Gas separation apparatus and method



Dec. 22, 1953 s, c. COLLINS GAS SEPARATION APPARATUS AND METHOD 2 Sheets-Sheet 2 Filed Sept. 21k, 1951 Patented Dec. 22, 1953 GAS SEPARATION APPARATUS AND METHOD Samuel C. Collins, Watertown, Mass.,` assigner to Joy Manufacturing Company, Pittsburgh, Pa.,

a corporation of Pennsylvania Application September 21, 1951, Serial No. 247,696

21 Claims.

This invention relates to apparatus for and a method of separating mixed gases, more specically being a methodand apparatus for the separation of air into a substantially pure product and a waste gas eilluent, herein oxygen and mostly nitrogen, respectively.

Where the mixed gases to be separated are those'which make air or a similar mixture, there always a problem in connection with the removal of impurities, which in the case of air are principally carbon-dioxide and water. Where the separation involves rectification at low temperatures, these impurities must be removed in order to prevent plugging of the apparatus by freezing of therimpurities at the low operating temperatures encountered. The impurities may be removed in any ofv a number of ways, but recent developments tend to move in the direction of condensing thel impurities out in reversing heat exchangers, in` which raw air flows through a heat exchanger in thermal contact with a cold product of the rectication, depositing the impurities on the surfaces of the passages through which the air ows; Periodically, the air and the cold gas exchange iiow passages, whereupon the cold gas removes the impurities by owing outward through the passage in which air has just been flowing, and the raw air ilows through the passage through which the cold gas has just been flowing.l Such a heat exchanger is called a reversing heat exchanger, and has substantial advantages over the removal of impurities Vby chemical means, these advantages being well understood by those skilled in the art.V

In spite of the advantages in the use of reversing heat exchangers, substantial problems are present, in that such reversing exchangers have a tendency to accumulate condensed impurities. Continued operation with the heat exchangers in which the impurities are not completely removedr eventually results inv almost complete plugging Vof the heat exchanger passages, necessitating shutting down of the appaf ratus topermit heating of the heat exchangers and the vaporization of the impurities.`

There is a type of very eiicient and compactV heat exchanger which has long been known to the art. This isV the Hampson type heat exchanger and has a reputation as an extremely elicient. and compact counter-flow heat exchanger. In a heat exchanger of this` type, many small bore longtubes, comprising a bundle, are coiled densely within a shell, thusforming a shell-and-tube type of heat exchanger. Most eiiicient use of a heat exchanger of this type for a high pressure gas, such as high pressure air,

and a low pressure gas, such as the returning In general, the low pressure gas would flow atY right angles to the tubes through relatively narrow slits between adjacent tubes, but of course variations of this arrangement are possible and workable.

When raw air must be treated in reversing exchangers for purification, the Hampson type is unsatisfactory, principally because the resistance to fluid flow is too great through the too small tubes. This would be true for thatzhalf of the complete cycle in which the low pressure .gas would have to return through the small tubes. The power lost due to pressure drop is a function of the fractional drop in absolute pressure. The purging gases are always at low pressure and are of relatively great volume. The actual drop in pressure resulting from a given mass rate of flow is proportional to the square of the volume. That fact, together with Vthe small absolute pressure, makes 'for intolerable power losses in such a reversing exchanger.` A possible remedy resides in using a suiiiciently large number of small tubes to avoid excessive power losses. However, such a solution would involve the use of such a large number (possibly thousands) of tubes as to make such a possible solution entirely impracticable.

It is accordingly an object of this invention to provide an apparatusand a method for the separation of gaseous mixtures like air containing impurities, in which reversing heat exchangers of the Hampson type can be used in a practical, efcient manner. This and other objects are accomplished in an apparatus in which two substantially identical shell-and-tube Hampson type heat exchangers are provided and are connected in the apparatus by means of suitable reversing valves. 'I'he reversing valves and the heat exchanger passages are so con-Y nected that the raw air containing the impurities and the purging gas always flows in the shell passages, which of course provide a low resistance to Aow.

It would not be necessary from the standpoint of power losses to have the air flow through the shell passages, because the air is at high pressure. However, because the low pressure return gases must ow through passages having a low resistance to ow and must 'flow' through the passages that are used by the air (alternately of course) in order to remove the impurities, the raw air must alsoflow through the passages having low resistance to flow. In view of this requirement, the ingoing raw air and the leaving low pressure gases are always passed through the shell passages of shell-and-tube type of heat exchangers; while other uids. at high pressure always pass through the tube passages. The high pressure fluid in a given tube passage is alternately a cold iiuid and a warm fluid, depending upon whether the tube passage must cool the raw air or extract refrigeration from the leaving low pressure and low temperature product. In addition, means are preferably pro vided to create a temperature drift`- in' the 'heat'V exchanger during that portion of the operating cycle in which impurities are removed.' by thel purging low pressure return gas.

In a preferred form of this invention, this temperature drift is brought about by the provision of means" to effectV a variation' in the flow of'warm gas through that; one off the reversing heat exchangers which is being purgedofim'- purit'ies.` The meansxreierred to involves the use of. a: variable flow device; More specically', another heat. exchanger has one passage thereof connected inparallel'with the tube passage of whichever one of! the reversing exchangers is being. purged. A. variable. restruction or orifice is placedi inl said. onei passage of.' the; other heat exchanger. By increasing the flow resistance' through'. the. two-passage heat` exchanger, a substantial'. overload of warm gas is forced'. through thei tube. passage or.` the reversing exchanger.;

This has. the. effect'. of: elevating the. temperature throughout the length. of: the heat exchanger. which. is being purged?. Since' theV carbon dioxide depositedonthe. exposed surfaces of the shelb passage, including the. outside` surfaces of these tubes, itsva-por pressure rises; as the: temperature rises. Inasmuch asfv a change of aslittle as ve degrees will have aprofound effect facilitating: removal of carbon dioxide bythe purging fluid',` thier change is;` signiica-nt The overload'- of warm gast should take place. for approximatelyv one-half to.v three-fifths of the4 half-cycle during which the4 heat exchanger isf purged.. For thelremainder: of. the half-cycle, the. variable restriction in. the fluid'- passage. of. the.' third heatl exchanger is removed, permitting the temperature to fall bach. again tothe tempera ture at whichV the heat exchanger operates. dur-- ing the other half-cycle.

In the draw-ings;

Fig. l shows a gas separation apparatus. according to. this invention.

Fig. 2. shows apossiblevariationof the Hamp-v son type of heat exchangers shown in Fig.. l.

Eig. 3Lis a ligure similar toFig.. l, but showing` apparatus. in which a. single rectication column is usedinplaceothe. double rectification column shown in...Fi'g. l.

made.-

The apparatus aspects of te invention Referring. now toV Fig.. 1 for a more specific. description and discussion of. the. apparatus aspects of the invention, I. have there shown a source of compressed. air 2 which. may be any suitable air compressor having an air intake 4 and anair discharges. The compressor. orsource of compressed air. preferably discharges through a suitable filter III (which is. provided for the removal ofi lterable impurities).

The iilter I0' is. connected' by means of' a conduit, I2. with a first reversing valve I4. Reversing changers HSA and valve I4 is connected With the reversing heat exchangers` IiA and IBB. More specically, the shell passages I8 and 20 of the reversing heat exchangers IieA and IEB are connected with the reversing valve I4 by meansof conduits 22 and 24 respectively. A waste gas discharge 26 completes the conduit connections with the reversing valve I4. Y

` It will of course be understood by those skilled in the art, that the passages in the reversing exchanger ISA and IB are two-ended passages. Exchangers. IfA and ISB are preferably placed with their warm ends down, as shown in the drawings. The Warm ends of the reversing ex- I-BB are connected with the conduits 22 and 2iI as aforesaid, and the other ends of the shell passages of the reversing exchangers IA and! IfB are connected with a second reversing valve 28 by means of conduits 30 andA4 32 respectively. Two other conduits, namely 34 and 36, complete the connections With the reversing valve'. 28..

A, third' reversing: valve 38.' is connected? with: the; reversing: exchangers iiAz. and'- IIiB' by wayK of" conduits All and 4.2: respectively. More speicifrcally; the Warm@ ends of: the tub'epassages 44 and 45 are connected with` the; conduits 40E andE 42: respectively. The other or' cold endsr ofi the tube passages 441 and 4B; are'. connected Witlr a4 fourth reversingA valvev 48' by means of conduits: 5) and 52 respectively..

The reversing valves |14, 28; Sii-and 418 may;4 all beV mechanically actuated; or, if desired, valves. Illand 38I may bemechanically actuated.v andi valves 2d and- 48: maybe reversing check valves as shown inmy' copending; patent. applica-tiene Serial No; 1261385-, led Mayv 13, 1950,v and as signed to theV assigneey of this. invention.

Itfwill of coursebeunderstoodhy those skilledl in the: art, that=tl'ie.-heat1exchangersv IGA and. IEBeare'shownschematicallyin'Fig. l. In this schematicshowing,` the tube'pa-ssages'44- and 45 arev shown' as' single tubes.. In actual practice, these singleA tube passages are preferably a. greatl many smalltubes all. connected withran inlet header and an outletrheader. As an. example of. a multiple-tube: passageA type of heat exchanger, I have'shown.. alsoV schematically, in Fig. 2. a. shell-and-tube heat exchanger of the Hampson type, inwhich atwo-ended shell passage 5G. has

its one end., connected With aA conduit 58. andits other endoonnectedwith alconduit Eil.. 'Ihetube passage or the heat exchanger 5.4 is provided by afplurality, here shownasthreeof small. passages 62, 62; and 62.. Itvv-illofl course-beunderstood-.by thoseskilled.. in the art,.that any desired number of plural passagesGZ, 52.', etc. to62," may be used. The one end, of, thev tube passage is connected with. a header 64 and the other endv is connected with. a. header 66.

Returning now to. the double.- column embodirnentshown inFig. 1A the, second reversing valve, namely valve 28, is. connected with. the conduits. 34 and 3I- as. aforesaid,l of. which.. conduit 34 is alwaysA connected to supply purified compressed air to the column` air inlet and the conduit 36 is always connectedto receive Waste gas effluent fromthe column Waste gas outlet, as will he seen below.v

In a preferred embodiment of the invention, asA shown here,A the entering puriiied. air and the leaving waste gas effluent are in heat exchange relation with. each other. To this end, a heat exchanger` 68 is provided having a passage 'is connected with the conduit' 34, and through which passage 'I0u entering airtherefore passes,

f and having another passage 12 connected with the conduit 36. Passage of heat exchanger 68 is also connected with theA column air inlet 14by`means 'of a'conduit 16. Passage 12 is connected with the column waste gas outlet 18 by means of a conduit 80.

The rectification column shown rin Fig. 1 is a conventional double column 82 having the usual air inlet 14 referred to above and a waste gas etlluent outlet 18. also referred to above. A conventional boiler-condenser 84 separates the lower column or high pressure section 86 from the upper column or low pressure section 88. A conduit 90 is provided to conduct oxygen-rich liquid air from the lower column section 86 and dis-y charge the liquid by wayof expansion valve 02 into the upper column-section 88. Similarly, a conduit 94 is connected to conduct nitrogen-rich liquid from the nitrogen shelf 96 and discharge that liquid into the upper column sectionby way of an expansion valve 91. The two liquids are preferably in heat exchange relation with each other in a heat exchanger 98 which has passages which may properly be considered part of the conduits 90 and04.

The column is alsoprovided with a product outlet, this beingv shown in the form of a stack type of outlet, or settling chamber, |00 of any suitable type to insure that only vapor leaves the column by way of the chamber |00 and the conduit |02 connected thereto. The product conduit |02 is connected with the shell passage |04 of a shell-and-tube heat exchanger` |06. A product supply conduit |08 is connected to receive warmed product from the passage |04.

The product supply conduit |08 has connected thereto another conduit 0 which discharges to atmosphere, conduit 0 having therein a valve 2. The product supply conduit |08 is also provided with a shut-olf valve ||4.

Returning again to the column 82, it can be seen that the boiler-condenser 84 has a small rare-gas bleed conduit ||6 connected with the upper end of the boiler condensercoil and connecting into the waste gas efuentconduit 80. Apparatus made according to this invention may if desired embody a refrigeration waster I8 which is simply a heat exchanger having a single passage therethrough and being con-` structed for heat exchange with ambient air. f

To this end, the refrigeration waster ||8 is connected by a conduit with the product collecting pot |22 in theevaporator-condenser section of the column 82. Preferably, the conduit |20 taps into the columnat approximately the normal liquid level of the column, the liquid referred to being the product oxygen.A Another conduit |24 connects the refrigeration waster with the product supply conduit |08. In the conduit |24, there is provided a valve |26 which is automatically controlled by a diaphragm |28,

one side of which is exposed tothe pressure at the upper end of the column by means'of a pressure conduit |30, and the other side4 of which` is exposed to the pressure in the lower end of,y

the upper column section by means of a pressure conduit |32. k

In order to bring the air down to the low temperatures at which rectication must be carried out, some suitable refrigeration source must be provided. This refrigeration source is the major source of refrigeration for the system, although of course some refrigeration is derived from lthe isenthalpic expansion .of air-.or vother fluids through the vseveral expansionzvalves.

(iii

In the embodiment of the invention shown herein, the source of refrigeration .is a compressor-expander circuit which utilizes gas having substantially the composition of the vapor above the liquid in the nitrogen shelf.

The refrigeration source includes the compressor shown at |34 and the expander shown at |36. The compressor |34 is a substantially conventional compressor suited to the purpose, and the expander |36 is preferably an expansion engine, or any other suitable device which can expand a fluid in such a manner as to extract internal energy from the uid, thereby lowering its temperature. To that end, the expander |36 may be any suitable type of expansion engine connected to an outside resistance. In a preferred form of the invention, the secondary compressor |34, the primary or air compressor 2, and the expander |36 vare all interconnected, eitherto the same shaft as shown schematically by the dotted lines |3iavand |31b, or are gear connected in any suitable manner to a suitable drive motor for the source of power for the system. Such a drive motor is not shown but Will ofcourse be understood by those skilled in -thc art to be any suitable source yof power such as an internal combustion engine, electric motor, or the like.

,The compressor 34 has a discharge connection |38 which connects with a suitable filter |40. Filter |40 is connected by way of a conduit |42 to the third reversing valve 38. In the conduit |42 there is provided a variable resistance or orice |44. The secondary compressor intake is connected with reversing valve 38 by means of conduits |46 and |48. Conduit |46 preferably has therein a variable orice or resistance |50.

The fourth reversing valve 48 is provided with connections to which the conduits 50 and 52 are connected as aforesaid, and in addition, con- ,Y duits |52 andr|54 are connected with the re-V Conduit |54 is connected with versing valve 48. another conduit |56, which in turn is connected to two branch conduits |58 and |80. The branch |58 is connected to one end of a iluid passage |62 in heat exchanger 88.VV The other end of fluid passage |62 is connected to a conduit |84 havingv therein alcombination expansion and shut-01T valve"|86. Conduit |64 is connected with the lower section 86 of the double column 82 byv nected with one end of a passage |84 in the heat exchanger 68. The other end of passage |84 is connected tothe waste gas eiuent conduit 36 by way of a conduit |86. Branch conduit |16 is connected to the conduit |52, and, through conduit |52, to the reversing valve 48. A conduit |85, having therein a check valve |88, is connected at its one end to the vapor space above the nitrogen shelf 96 and at its other end into the branch conduit |16 downstream of the valve |80.

Considering -now that shell passage 20 forms a first iiuid passage, tube passage 46 forms a f second iuid passage, tube passage 44 forms a third 'fluid passage, and shell passage |8 forms azfouiitlr fluid passage, we have novo a fth lil-uid passage. M85, wh'clris the tube passage inf the hea-t exchanger mi; the. passage: |i|l4=v constitut-I ing; asixth fluidi passage. The fifth. passage |588 is connected in; parallel with whichever' one of the third or second passages, 44 andi 4|?Y respectively; happens to` be connected forv the cooling ofl compressed gas. The connection` of41 heat exchanger passage |88 in parallel with the thi-rd or second passage 44`- or 452 isA accomplished by a conduit |190 having therein a variable orifice or resistance |92`f. Accordingly, conduit |99 connects one end oifiluid passage |88 with the conduit |42. The other end ofv fluid passage |88= is connected with; conduits i541 and llbymeans oi a conduit |94.

lf desired, still other fluid passages may be provided tor the cooling of the hot compressed. gas and for the heating of any excess of cold exhaust gas from the expansion` eng-ine as may be left over. vTo this end, a fluid-passage |98 is provided in parallel withA the *fl-uid passage |88, by having connected tov one end' thereof' a conduit |593` which connects with conduit` |94 and` to its other enda conduit Mil-whichconnects'withY conduit |96'. A variable resistance- 2'02 is providedin the conduit 29|?. A fluid passage 2&4 inheat exchange relation with iluid passage it in a heat exchangerV 206 is; connected at its one end with conduit |52 by meansof a conduit 288 and at its other end with conduits |46 andY |58 by means of a conduit 210. A variable resistance 2|2" is provided in the' conduitl 2||J'. is preferably thel tube passage, and the passage 24M the shell passage, of a shell-and-tube type of heat exchanger.

Variable resistances- |92 and 262 will be con'- nected in any suitable` manner to close during the nrst portion ofy a half-cycle of the reversing heat exchange cycle. In other words, just as thereversing valvesV Ul,l 28, 38 and 48 operate, the variable resistances:` |92 andA 202L will be tightened' orl closed somewhat. At a point approximately midway to three-fifths of the completion of the half-cycle, the variable resistances |92 and 202' will be opened again to apredetermined position. The variable resistances |144", |553.,` and 2|2l will be used to securel the. desired now and heat balance among the; various'fluids involved.

Operation of the variable resistances |92j and 262 as described above may be accomplished in any suitable manner as will be, understood' by those skilled in the art. Thus the valvesl |92 and 22 may be controlled by a cam driven by the same shaft that controlsl the reversing valves` |13, 23, 39 and 48. v

Reference` will now be had to Fig. 3 for a description of the embodiment shown therein. The embodiment shownin Fig; 3 diiersfromY the emboclimenty shown in ratus shown in Fig. 3 employs a single column whereas the apparatus shown inl Fig. l employs a double column. Accordingly, whereas in Fig. .1. the compressed airline 'l going tothe double column opens into a liquid pot in the lower section of the double column, the corresponding line 16 in Fig. 3 goes instead to one end of a coil 2|:6 in the boiler-condenser 2|8 located in the bottom of the singl'efcolumn 220'. The other end of coil 2|6 is connected to a conduit 222V which is connected to the upper end of the column at 224. The fluid flowing in the conduit-221l isA in thermal contact with the waste gas effluent in a heat exchanger 226. Conduit.

The passage |98" Fig; 1 only in that the appa- 222', on'v the downstream side of heat exchanger 22,6, is: provided with anexpansion` valve 2228. To the waste gas outlet T3', therel is secured a waste gas conduitA corresponding to the conduit Sil-of Fig. 1. Conduit-8U passes through the heat exchanger 226 to provide the aforesaid thermal contact-v with the air in conduit 222'. At a point downstream of heat exchanger 226i,v conduit 80 is connected to a passage 72' of: al'ieaty exchanger 68', which correspond respectively tol the passage 'I2' and the heat exchanger 68 of the Fig. 1 embodiment.

Another difference in the twol embodiments liesy in the fact that, whereasv in the embodiment shown in Fig. l, the line |64 discharges intothe nitrogen shelf of the double column boiler-condenser, in the embodiment shown in Fig; Sthey corresponding line |64 discharges into the air'` line TSG by way of a valveV |66. Also, whereas-inl Fig. 1 the line |85 connects through a check valve |33 with thev vapor space above thev nitrogen shelf, in the embodiment shown in Fig. 3; the make-up line is connected through checkr valve |536" with the airline 16|'. i

In other respects, the embodiments shown inr Fig. l1 and Fig. 3 will be found tobe substantially identical so the remainder of the` apparatus shown in Fig. 3 will not be described in detailV here.

The method aspects. of theimzentiom For a better understanding of the meth-od aspects of the invention, reference can behad to Fig. l which shows apparatus in which the method according tothis invention may be prac-1Y ticed. As there shown, raw compressed air passes through a first passage (which is the shell passage 2li) in heat exchange with a coldl fluid now-- ing in a second passage (the passage 461 shown in Fig. l).

As the raw air passes through the rst passage referred to' in exchangewithcold uid in the second passage, the air is cooled and the condensable' impurities, such as the carbon dioxide fand the water, are condensedon the cold surffaces` ofV the passage through whi'chthe air is owing, the carbon dioxide condensing of course in the solid state, and the water condensing in both the liquidl and the solid states. Air leaving this passage goes to a rectication column for rectification into a substantially pureproduct and a waste gas eilluent. The air could flow: directly to the column for the indicated purpose, but in the preferred form of practicing the method aspect of the invention, the air is further cooled prior to rectication byy heat exchange with another cold fluid', which is conveniently and preferablyI the waster gas eili'uent from the rectification process. Thus the air leaving therst passage referred to is in heat exchange` with waste gas eiiluent'v in a. heat exchanger S91 and passes from this heat. exchanger into the column Where it is rectified into the pure or substantially pure product, in this case oxygen, and a remaining constituent which comprises a waste gas effluent, mostly nitrogen.

The ow in the passages 20 and 46 takes placey over a predetermined time through a predetermined length of half-cycle. Thislength of halfcycle may conveniently be veminutes, although of course other operating periods may be adopted and may be adapted to-'different types of apparatus. At the end of the-halfz-cycle reerred to, the. air and cold fluid are sent through other similar passages, whereupon warm fluid is. put:

through the passage through which cold uid passed, and waste gas eiiiuent is put through the passage through which air passed. The warm uid heats the surfaces on which the impurities were condensed during the previous half-cycle and vaporizes them, and the outiiowing waste gas carries the impurities out along with it. Thus. in the heat exch-anger |6B, the warm gas in the passage 46 heats the surfaces and vaporizes the impurities, andthe thus vaporized impurities are swept out of the passage by the leaving Waste gas eiiluent owing in that passage. At the end of the second half-cycle, fluids owing in the first and fourth passages exchange passages and the fluids owing in the second and third passages exchange passages at the same time. In the apparatus shown in the embodiment in Fig. 1, this change is accomplished by means of the reversing valves shown schematically as valves |4, 28, .3B-and 48.

In order to accomplish the described rectification at the low temperatures necessary for a gas such as air, refrigeration has to be supplied from a suitable source, which may be any compressorexpander type of refrigerator. Such a suitable source is shown in the compressor-expander circuit indicated schematically in Fig. 1. In the embodiment there shown, the fluid circulated in thei compressor-expander circuit tends to lbe of the same composition as the gas in the vapor space above the nitrogen shelf of the double column, because of the fact that the make-up line |85v for thecompressor-expander circuit is connected into the vapor space of the nitrogen shelf.

The uid flowing in the refrigeration circuit is compressed in the compressor |34 and flows in the aforesaid third passage 44 in heat exchange with a cold fluid owing in the passage I8. In the course of this heat exchange, the warm uid in the passage sublimes and vaporizes the previously condensed carbon dioxide and water which'are then swept out of the space |8 by the cold outgoing waste gas eilluent and are carried out to the atmosphere. The heat exchange referred toas taking place in the aforesaid third and fourth passages, takes place during a half-cycle of operation, at the end of which time, the warm gasexchanges passages with the cold gas in the passage 46. In other-words, the warm gas then goesthrough passage46 and the cold gas through passage 44.

.Warm gas is cooled in the passage 44 and leaves the heat exchanger l 6A in a cooled condition, passing thereafter to the expander |36 wherel it expands with the performance of external work and Vis lowered in temperature. The cold exhaust gas leaves the expander |36 and is the cold iiuid referred to above as passing in heat exchange relation with the ingoing raw air. This cold fiuid passes through the passage 46 referred to'above and returns in,` a warm condition to the compressor intake. v

Preferably, the flow ofy the Warm gas' in`its passage in heat exchange with waste gas efliuent is varied during the half-cycle of operationin which time. it passes through its channel in heat exchange with the waste gas eliluent. This channel will be the third or second passage depending upon which one of the passages happens to be connected with the gas compressor discharge.

Apreferred manner of practicingfthe invention is to increase the ow of warm fluid in the passage referred toduringthe first one-half to threefifths', of. a half-cycle.

halfcyc1e,-thefiow ofwarm gas is greater during Y Thus, for a ve minute the first three minutes of the half-cycle than it is during the remaining two minutes thereof. The object of this increased flow is to warm up the passage (especially the cold end of the passage) through which the warm gas flows. It will be remembered that the outside surface of this pas.- sage is the surface upon which the impurities have been condensed out of the air during'the previous half-cycle of operation, vIt is therefore advantageous to increase the temperature of the surfaces during the rst portion of the half-cycle of operation, thereafter reducing the quantity of warm gas flowing through the passage in order that the heat exchanger may be returned again to its normal cold temperature in preparation for the next half cycle.

The phenomenon referred to, which may for convenience be called a temperature drift, is accomplished in the preferred embodiment shown in Fig. 1 by means of a bypass uid passage connected in parallel with the aforesaid third or second passage, depending upon which one of the passages happens to be connected.

The bypass passage referred to is preferably provided in the form of a fifth uid passage, clisposed in heat exchange (in heat exchanger |06) with cold product returning through a sixth uid passage. The flow variation referred to during the course vof a half-cycle is accomplished by placing a variable resistanceL or orifice in the fifth fiuid passage referred to. The preferred practice involves an increase in the resistance to flow in said fifth fluid passage during the first portion of the half-cycle, in order thatincreased ow in the third or second passage, whichever happens to be connected, may take place. Thus, a variable resistance |92 is provided in the line |96 which is connected with the passage |88. In the preferred method of practicing the invention, the variable restriction |92 is controlled by the sages.

In the preferred embodiment shown in Fig.v 1, still another fluid passage is connected in parallel with the fth fluid passage referred to, this connected conduits) shown in the` apparatus of It will of course be understood by those skilled in the art, that a resistance 202 similar to the resistance |92 is provided in the conduit 200 connecting with the fluid passage |96, in order that the above-described means of varyingfluid flow in the second or third passage as described panded gas flowing in the passage 204 being the excess over and above lthe quantity of gas which 1s required in the second or third uid passage.

ited extent, the advantages of the invention may be practiced even without the heat exchanger |06 and its passages |88 land |04 referred to.

For a better understanding of the method aspect lof the invention (and, for that matter, also Y to a pressure of 85 p. s. i..g. and

first `fluid passage 20 at saidv pressure of 85 p. s. i. g.

`125cc-Syler `11 'the apparatus aspect) "a few approximate values may now be provided in thenature of temperatures and pressures for the various portions of the apparatus and circuits.

Air is compressed in the primary compressor 2 thus enters the and at `ternp'erature. of 310 K.i. e., atmospherio temperatire.- Air leaves the passage 20 at .a pressure of 82 p. s. ifgrand atemperature of 115? K., .entering the passage Vlt! of. heat ex- 'changer4 68 in that condition. Air leavesathe `passage 1|)A state. it enters the bottom or lower section of the double column, The air thus entering the rectiflcation column is substantiallyientirely vapor. Approximately ve percent .by weight may beliquid at .this time, butthis will be a very negligible .percentage by volume. Oxygen-rich liquid air leavesthe lov-Jer. :section of the doublecolumn by way. of. .conduit at the aforesaid 80 p. s. i.1g. and at 102 K., 'passing to the heat exchanger '98. The oxygen-.rich liquid air leaves heat exchanger Sfat the 80. p. s. i. g. referred toabove and at 90 K. and expanded in the expansion valve 9.27going, intov the upper. section of the double .Columnat approximately 'l p. s. i. g. Waste gas eiiuent leavesthe column by way of conduit 80 at s,..i. g..and 82 enteringthe heat exchanger. 6,8.W aste 'gas eluentle'aves the heat exchanger 68.at 5 p. s. i..g..and at 106 entering the reyersing heat vexchanger in that state. Wastegas effluent lleaves the reversing heat ex- Changenat..Substantially atmospheric pressure vand atmospheric temperature, or approximately '71.K.. Oxygen `roduct leaves the column by way of the connectQn I jjandconduit |02 at '7 p. s, i. g. andi" K andleaves .the heat exchanger |06 at substantiallyatmosphericA pressure and 307 K.

Nitrogenleaves the nitrogen shelf by way of con- Quit at 80*p.si.ig..and 97.5 K., entering the A after expanding to l0 p. s. i. g.,

heat $21.6...1lg11 98 an thus entering heat exchanger 98 at 10 p. s. i. g.

andu83b `Nitrogen;rich,,liquid and vaporleaves l.

llsllbsatsxshaeser 8.8 atras. rg. and 82 K.

th top of the\o lumn sd Vin the secondary compresapsing fromlGO nfs. i. e. to 290 ngi enters thel third fluid passage reovev jgfhepressure indicated and at i@ 'it pasture... The refrigerant. gas '1;,1251E andai. 199 p. erg.

.el .hisl1.p;ressure of 20ol v 'SO-Oled.einpressedgas.passes, to

der" f3@ and leaves .the .expander Yat 80 'entering the second fluid H A w at state.A Theexpanded. gas he passage'qnatu' p. s.i. g .,ar1 l1 305 K.

15 I. .Ves

ande' 'ers"the`secondary compressor intake in ncrea'sed, necessitating an infat 80 p. s. i. g. and 103 K., in whichr throughthe thirdp'assage causes the tempera;

ture of reversing heat exchanger I'BA'to begin 'to rise during th`e"half`cycle in which warm gas passes `through its ,tlbage or passages. This temperature drift continue'sfuntil valves |92 and 202 are returned to normal at the endoftlie three-,minute period, whereupon the tempera# ture begins to settleragain to what it would be without the temporary 'restriction In mathematical terms, .the operation may be described by stating that the flow ratio Third'or second passage Fifth passage varies during the time period for which the'third or second passage isiconnected to receive warm, compressed uid,.that time `period beingahalfcycle in the embodiment shown.

Y' vIn.allition to the drift referred to,'there isan incidental `temperature i drift caused by the fact that the quantityof'fluidcirculated inthe coinpressor-expander circuit isslightly more `Athan the minimum4 required to eiect the V"necessary cooling of raw `,air toy accomplish substantially a complete deposition v*of carbon-dioxide. This slight excess of` compressed refrigerant fluid will bringaboutla downward drift (lowering) of the temperature of afreversing heat exchanger when it is receivingujcold expanded fluid; and upward when itisvreceiving warm compressed nuid.

lhetemperaturedrift (of both types) referred to aids in the removal of impurities, especially the.:carbonjdioxide.`J 1110;` begin with, the "downwarddrift graduallyshifts the location of `the Zone ofxlowest temperature, Aand therefore of course also effectsma gradual and continuous shift along .the oi/ n zsidel o f the tubes ofthe carbonedioxide 'deposition zonemjlhe resultis'- a 'more effective removal of impurities from the air. The upwarddrift ofternperature operates "of course. ir 1 the y reverse manner, bringing about a gradual shift along the tube surfaces of the maximum'tem Yattire, resulting inna more eiective removalof impurites from" the reversing -heat -exchanger.

. 'SClpiiQn ofthe operationforicoolinglnownthe apparatus from. room. temperature to operating temperaturesWToj prepare the apparatus :for startingthewoperatoropens valves |12, |66" and |82 andfmclosesvalves ||4 and |80. During'the cool-,down period, the valve |66 operates 'asan expansionvalve. 1 l Air e ters the' primary compressor byway"of theintalge d and is compressed, passing through reversing valve .HlA tqthe first passage 2|J` offli5at exchanger IBB. The air leaves the'passage'20'a'nd heat exchanger ISB by way of reversing valve 28, passing to the passage in heat exchanger 68, and thence going to the column by way of conduit 16. v f

In the column, air flows to the upper section 88 of the column by way of conduits 90 and 94. At the same time, however, air also leaves the lower section of the column by wayr of conduit |85 and check valve |86. This air goes to the line |16 which is connected with the secondary compressor intake by way of fluid passages 46 and 204. Air thus enters the secondary compressor and is compressed, leaving the compressor I 34 by way of conduits |38 and |42. A portion of the air passes to the reversing valve 38 and from there through heat exchanger IBA, namely passage 44 thereof, leaving the heat exchanger |6A by way of conduit 50 and going to reversing valve 48, which it leaves by way of the conduit |54. Meanwhile, the remainder of the air from the secondary compressor passes by way of conduits |90 and 200 through passages |88 and |96 respectively and rejoins the air from the passage 44 in the conduit |58, passing to the expander |36 by way of conduit |60. Air enters the expander by way of the intake |10 and is expanded therein at the expense of its internal energy, leaving the expansion engine by way of exhaust |12 at a reduced temperature.

The cold exhaust passes to conduits |14 and |18, thence to passage k,|84 in heat exchanger 68, and from there to the conduit 36 and reversing valve 28. Thence the exhaust, now partially warmed by heat exchange in the heat exchanger 68 with ingoing air, leaves the reversing valve 20 and passes to the passage |8 of heat exchanger |6A by was7 of conduit 30. It will be understood that the expansion engine exhaust joins the air leaving the column by way of the efuent conduit 80 and passage 12. The vairfrom the eiliuent or waste gas outlet 18 of the column and the expansion engine exhaust pass through passage I8 of-heat exchanger IBA in heat exchange with the secondary compressor discharge air, and leave the system by way ofthe discharge conduit 26 and reversing valve I4.

Simultaneously, air is flowing in the passages of heat exchangers 206 and |06, leaving the product line |08 by way of valve ||2 and conduit H0.

The heat exchange in heat exchanger |6A (or |6B depending upon the position of the reversing valves) between the expansion engine exhaust and the compressor discharge, results in regenerative cooling, which eventually brings the air temperature down low enough so that liquefaction of the air begins to take place in heat exchanger 68. Liquid gradually begins to build up in the lower section of the column, and is carried to the upper section of the column. The operator watches the liquid accumulation in the oxygen pot |22, andwhen the liquid level has reached a predetermined desired point, the operator opens valves ||4 and |80 and completely closes valves ||2, |66 and |82. 'Ihe apparatus is thereupon ready to produce substantially pure oxygen product.

It will of course be understood by those skilled in the art that, at the very outset, the fluid flowing in the compressor-expander circuit comprising the secondary compressor |34 and the expander |36 is air. However, as the system graduelly cools down and enters the normal operating phase, the compositionr of the fluid flowing in the compressor-expander circuit will gradually change until it is substantiallyA nitrogen, inasmuch as the make-up connection for the com presser-expander circuit is directly to the vapor space above the nitrogen shelf in the evaporatorcondenser in the double column. In the apparatus shown in Fig. 3, the make-up gas connection is to the compressed air line 16', so the refrigerant fluid tends to be air, possibly enriched somewhat in nitrogen because of some liquefaction in heat exchanger 68', the liquid there forming being rich in oxygen.

Operation of the apparatus after cool-*down is completed will be well understood by those skilled in the art from what has already been set forth, and any further discussion of the operation would merely result in unnecessary repetition.

A brief description of the operation of the refrigeration waster is in order, because under certain conditions the use of a refrigerator waster may be desirable. This feature of the apparatus is particularly useful when separating air having impurities which are not satisfactorily removed in a heat exchanger.

The refrigeration waster serves the purpose of a continuously operating purge line, because some of the liquid product containing impurities is being removed all the time. Inasmuch as the constant boiling of the liquid product tends to carry solid impurities to the top, a continuous removal of a small percentage of liquid from the surface prevents the concentration of impurities in the boiler-condenser. The liquid product containing impurities is thus drained off and vaporized in the refrigeration waster. The product itself is not lost because the refrigeration waster discharges into the product line |08 by way of conduit I 24.

The quantity of liquid thus drawn oil may vary, but will ordinarily not exceed ve or six percent. Flow through the refrigeration waster is controlled by the valve |26, which in turn is controlled by the pressure drop from bottom to top of the column (Fig. 3), or of the low pressure section of the column (Fig. 1).

It will be seen from the foregoing that I have here provided apparatus and a method for the separation of gases which effectively removes impurities from the mixture prior to its rectification, and which discharges the impurities toatmosphere effectively and prevents the accumulation thereof in the apparatus.

While this application specifically describes apparatus and a method for practicing the invention, it will be understood that these are set forth for purposes of illustration, and that the invention may be modif-led and practiced in other ways without departing from its spirit or the scope of the invention.

I claim:

1. In an air separation system having a source of compressed air, a rectification column having an air inlet and a waste gas outlet, a source of refrigeration, and a waste gas discharge, the improvement comprising: a pair of reversing heat exchangers, each of which has a fluid passage comprising at least one tube and a different fluid passage comprising a shell surrounding said at least one tube, means to pass air from said air source through the shell passage of one of the reversing heat exchangers to the column air inlet, means to pass a cold fluid through the tube passage of said one of the reversing heat exchangers whereby condensable impurities are deposited out of the air onto surfaces disposed in the shell passage of said one of the reversing heat exchangers, means to pass column waste gas from disposed in the shell passage of said one of the reversing heat exchangers; means'to pass column waste gas from the column waste gas outlet through the shell passage of the other one of the reversing heat exchangers to the Waste gas discharge; means to pass a Warm iiuid through the tube passage of said other one of said reversing heat exchangers whereby previously condensed impurities are removed from the surface of said other one of the reversing heat exchangers and are carried out though the waste gas passage; means, including all of the aforesaid passing means, periodically to interchange the passages through which the mixture and the column waste gas flow and at the same time to interchange the passages through which the cold and warm fluids flow; another heat exchanger having two fluid passages; means to connect one of the uid passages of said another heat exchanger in parallel with the tube passage of said other one of said reversing heat exchangers; and means to connect the product outlet to the other iiuid passage of said another heat exchanger.

13. In a gaseous mixture separation system having a source of compressed mixture; a rectication column having a mixture inlet, a product outlet, and a waste gas outlet; a source of refrigeration; and a Waste gas discharge; the improvement comprising: a pair of reversing heat exchangers, each of which has a iluid passage comprising at least one tube, and a different fluid passage comprising a shell surrounding said at least one tube; means to pass mixture from said mixture source through the shell passage of one of the reversing heat exchangers to the column mixture inlet; means to pass a cold iiuid through the tube passage of said one of the reversing heat exchangers whereby condensable impurities are deposited out of the mixture onto the surfaces disposed in the shell passage of said one of the reversing heat exchangers; means to pass column waste gas from the column waste gas outlet through the shell passage of the other one of the reversing heat exchangers to the Waste gas discharge; means to pass a warm uid through the tube passage of said other one of said reversing heat exchangers whereby previously condensed impurities are removed from the surface of said other one of the reversing heat exchangers and are carried out through the waste gas passage; means, including all of the aforesaid passing means, periodically to interchange the passages through which the mixture and the column waste gas flow and at the same time to interchange the passages through whichthe cold and Warm uids ow; another heat exchanger having two uid passages; means, including a variable ow device, to connect one of the fluid passages of said another heat exchanger in parallel with the tube passage of said other one of said reversing heat exchangers; and means to connect the product outlet to the other fluid passage of said another heat exchanger.

14. A method for'the separation of a gaseous mixture by rectification into its constituents to produce a substantially pure product and a waste l gas eilluent, comprising expanding a iluid with the performance of external work to produce a cold exhaust fluid, cooling the mixture in a rst passage by heat exchange with said cold huid flowing through a second passage to effect the removal from the mixture of condensable impurities, rectifying the mixture as aforesaid, compressing the iiuid after said heat exchange with the mixture, cooling at least some of the compressed fluid in a third passage by heat exchange with the waste gas eiiluent flowing through a fourth passage, whereby the compressed iiuid vaporizes previously condensed impurities and the waste gas effluent carries the impurities out, thereafter expanding the fluid as aforesaid, and periodically interchanging the flows in the first and fourth passages and at the same time interchanging the flows in the second and third passages.

15. A method as in claim 14, in which compressed iiuid is also cooled in a fifth passage by heat exchange with cold product iiowing in a sixth passage'.

16. A method as in claim 15, in which the cooling in the fth passage is effected simultaneously with the cooling of the compressed iiuid in the third or second passage according to which one is connected, and in which the flow ratio compressed iiuid.

17. A method as in claim 16, in which said ratio is greater at the beginning of said period than at the end thereof.

18. A method as in claim 17, in which compressed uid is also cooled by heat exchange with cold fluid at the same time that the other cooling of the compressed fluid takes place.

19. A method as in claim 18, in which, after said heat exchange in a first passage, the mixture is rfurther cooled by heat exchange with the waste gas eiiluent, which further cooling of the mixture preheats said effluent prior to its heat exchange with compressed uid.

2o. A method as in claim 14, in which compressed uid is also cooled by heat exchange with cold fluid at the same time that the other cooling of the compressed fluid takes place.

21. A method as in claim .14, in which, after said heat exchange in a i'lrst passage, the mixture is further cooled by heat exchange with the waste gas eiiluent, which further cooling of the mixture preheats said eiiiuent prior to its heat exchange with compressed fluid.

SAMUEL C. COLLINS.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,842,263 Gobert Jan. 19, 1932 2,526,996 Crawford Oct. 24, 1950 2,534,478 Roberts Dec. 19, 1950 2,584,381 Dodgev Feb. 5, 1952 

